In view of recent global environmental issues, emission regulations have been tightened for automobiles, and reducing the weight of automobile bodies has been a critical issue.
To reduce the weight of automobile bodies, one effective approach is to reduce the thickness of steel sheets by increasing strength (sheet metal thinning). As a result of remarkable advancement of steel sheet strength, there is an increasing trend toward the use of thin steel sheets whose thickness is less than 2.0 mm, yet TS is not lower than 780 MPa. Such sheet metal thinning, however, can be accompanied by a reduction in vehicle body's rigidity, which is considered problematic, and further improvement in the rigidity of structural parts of automobiles is becoming an issue. The rigidity of structural parts is determined by the thickness and Young's modulus of the steel sheet for the same cross-sectional shape. Accordingly, it is effective to increase the Young's modulus of the steel sheet to achieve reduction in both weight and rigidity of structural parts.
The Young's modulus of a steel sheet is largely controlled by its texture, and in the case of iron, which has a body-centered cubic lattice, the Young's modulus is known to be high in the <111> orientation, in which atoms are densely packed, and low in the <100> orientation, in which atoms are less densely packed. It is known that the Young's modulus of ordinary iron having no anisotropy in crystal orientation is about 206 GPa. If anisotropy is given to the crystal orientation to increase the atomic density in a specific direction, it is possible to increase the Young's modulus in that direction. For the rigidity of an automobile body, however, as loads are applied from various directions, it is necessary to set a high Young's modulus not only in a specific direction but also in every possible direction.
On the other hand, increasing the strength of the steel sheet leads to deterioration of formability. It is thus difficult for a steel sheet to have both increased strength and excellent formability. Therefore, it is desirable to develop steel sheets with increased strength and excellent formability.
In response to such a demand, for example, JP2007092130A (PTL 1) proposes “a method for producing a high-strength thin steel sheet with high rigidity, the method comprising: hot rolling a slab to obtain a hot-rolled steel sheet, the slab comprising a chemical composition containing, by mass %, C: 0.02% to 0.15%, Si: 0.3% or less, Mn: 1.0% to 3.5%, P: 0.05% or less, S: 0.01% or less, Al: 1.0% or less, N: 0.01% or less, Ti: 0.1% to 1.0%, and the balance consisting of Fe and incidental impurities; cold rolling the steel sheet at a rolling reduction of 20% to 85%; and subjecting the steel sheet to recrystallization annealing to have a ferrite single-phase microstructure, a TS of 590 MPa or more, a Young's modulus of 230 GPa or more in a direction at 90° with respect to a rolling direction, and a mean Young's modulus of 215 GPa or more in directions of 0°, 45°, and 90° with respect to the rolling direction”.
JP2008240125A (PTL 2) proposes “a method for producing a high-rigidity and high-strength steel sheet with good formability, the method comprising: hot rolling a slab to obtain a hot-rolled steel sheet, the slab comprising a chemical composition containing, by mass %, C: 0.05% to 0.15%, Si: 1.5% or less, Mn: 1.5% to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Nb: 0.02% to 0.15%, Ti: 0.01% to 0.15%, and the balance consisting of Fe and incidental impurities; cold rolling the steel sheet at a rolling reduction of 40% to 70%; and then subjecting the steel sheet to recrystallization annealing to have a mixed microstructure of ferrite and martensite, a TS of 590 MPa or more, and a Young's modulus of 230 GPa or more in a direction orthogonal to a rolling direction”.
JP2005120472A (PTL 3) proposes “a method for producing a high-strength steel sheet, comprising: hot rolling a slab to obtain a hot-rolled steel sheet, the slab comprising a chemical composition containing, by mass %, C: 0.010% to 0.050%, Si: 1.0% or less, Mn: 1.0% to 3.0%, P: 0.005% to 0.1%, S: 0.01% or less, Al: 0.005% to 0.5%, N: 0.01% or less, Nb: 0.03% to 0.3%, and the balance consisting of Fe and incidental impurities; then cold rolling the steel sheet; and subjecting the steel sheet to recrystallization annealing to have a steel microstructure with an area ratio of ferrite phase of 50% or more and an area ratio of martensite phase of 1% or more, and to have a Young's modulus of 225 GPa or more in a direction orthogonal to a rolling direction and a mean r value of 1.3 or more”.
JP2008240123A (PTL 4) proposes “a method for producing a high-strength thin steel sheet with high rigidity and good hole expansion formability, the method comprising: hot rolling a slab to obtain a hot-rolled steel sheet, the slab comprising a chemical composition containing, by mass %, C: 0.05% to 0.15%, Si: 1.5% or less, Mn: 1.5% to 3.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.5% or less, N: 0.01% or less, Nb: 0.02% to 0.15%, Ti: 0.01% to 0.15%, and the balance consisting of Fe and incidental impurities; cold rolling the steel sheet at a rolling reduction of 40% to 75%; and then subjecting the steel sheet followed by recrystallization annealing to have a microstructure with an area ratio of ferrite phase of 50% or more, and to have a TS of 590 MPa or more, satisfying a relation of TS× hole expanding ratio λ≥23000 MPa·%, and a Young's modulus of 235 GPa or more in a direction perpendicular to a rolling direction”.